Flexible duct insulation made with a rotary fiberizing process, must pass flame penetration testing in which a 24″×24″ piece of finished duct is exposed to open flame in the top center portion of a furnace at a nominal temperature at the start of testing of 1425° F.±25° F. The target temperature is measured by an array of thermocouples mounted in the center of the test apparatus with refractory insulation covering the top of the gas furnace. Once the target temperature is achieved in the center of the top part of the furnace and held in this range for 15 minutes, the refractory cover is removed and is replaced with the test sample, attached to a 24″×24″ angle iron frame. An 8-pound test weight with a surface area exposure of 1″×4″ is set on the top surface of the test sample, at which time the test timing is started. The sample must resist any visible flame penetration as well as support the test weight without it falling through the sample for a minimum of 30 minutes to pass the test. Three consecutive samples must pass the flame penetration test in the test apparatus described above for product to be certified by Underwriters Laboratories (UL) as meeting the flame penetration test requirement.
Test failures may be caused by a combination of multiple factors, including, for example, product density, product density distribution, fiber length, fiber diameter, glass chemistry and properties, design of the duct, as well as other factors. Improvement in any of these areas to provide a duct insulation which meets all test requirements would be of great value to the industry. To help support the test weight, a woven scrim of e-glass fiber is often used as a part of the flexible duct product. The woven scrim of e-glass fiber is undesirable because of material and duct fabrication costs.
Current glass often used to manufacture current duct insulation has a log-3 viscosity temperature in the range of about 1820° F. and a softening point temperature of about 1200-1210° F. As used herein, “log-3 viscosity” refers to the temperature at which the viscosity of a glass is 1,000 poise. This viscosity is representative of the fiber-forming viscosity and hence the temperature represents the approximate operating temperatures of the rotary fiberization process. Softening point is defined by ASTM C338-93 (2003) as the temperature at which a fiber with an average diameter in the range of 0.55-0.75 mm (+0.01 mm) and length 235 mm (±1 mm) will elongate under its own weight at a rate of 1 mm/minute. This corresponds to an approximate viscosity of 107.60 to 107.65 poise. Low softening point temperature in conventional glass is believed to contribute to flame penetration test failures. Generally, a glass composition change that increases softening point also increases log-3 viscosity by a similar or greater magnitude. Liquidus is defined by ASTM C829-81 (2005) and in ASTM C162-05 as the maximum temperature at which equilibrium exists between molten glass and its primary crystalline phase. Effectively, it is the temperature at which the glass devitrifies, or first forms crystals, undesirable in glass fiber-forming processes.